Optical systems used in laser printers may be characterized as having three sub-systems or assemblies, namely, a laser diode/pre-scan optical assembly, a scanning assembly, and a post-scan assembly. Typically, the laser diode/pre-scan optical assembly includes a laser diode emitting a diverging laser beam, a collimator lens for collimating the beam emitted by the laser diode, and a pre-scan lens to focus the beam to a waist near the scanning device so that the post-scan assembly images the beam to a waist at a corresponding photoconductive (PC) drum surface.
The scanning assembly generally includes a scanning device such as a motor driven, rotatable polygon mirror having a plurality of peripheral mirror surfaces or facets that rotate during operation of the printer. The mirror surfaces reflect the collimated and focused beam received from the laser diode/pre-scan optical assembly. The direction of rotation of the polygon mirror determines the scan direction of the beam passing along a scanned object, such as a PC drum in a laser printer.
During a calibration procedure, the collimator lens must be properly aligned so that the beam exiting the collimator lens is properly position in process and scan directions. Further, a pre-scan lens must be properly positioned so that the laser beam waist is substantially located in the process direction, transverse to the scan direction, at the surface of a corresponding PC drum.
In a known laser diode/pre-scan optical assembly for a laser scanning unit, supporting structure is provided for mounting pre-scan lens structure comprising a pre-scan lens, and a laser diode/collimation assembly structure comprising laser diode and a collimator lens. The supporting structure is formed with precisely located mounting points held to close tolerances which accurately align the pre-scan lens structure and laser diode/collimation assembly structure to produce a laser beam output spatially oriented to a predetermined location and including the formation of a beam waist at a desired point relative to a corresponding PC drum surface. A certain degree of alignment of the pre-scan lens is provided by taking a physical measurement of the position of the pre-scan lens or its carrier relative to a reference feature formed on the supporting structure. While such an alignment procedure provides relatively accurate alignment of the pre-scan lens, it does not compensate for variations in the beam resulting from variations in the physical characteristics of the pre-scan lens, such as minor variations in thickness or aberrations in the pre-scan lens which may affect the character of the beam passing therethrough. Typically, the collimation assembly is merely secured to the housing without any alignment steps being performed. However, a known alignment procedure, performed as a repair operation, involves directing a video camera to a downstream optical component, such as a mechanical target, so that the location of the beam spot can be visually observed by an operator on a monitor. The operator then installs shims as necessary to the mounting points for the collimation assembly to locate the beam at a desired observed location on the mechanical target.
Accordingly, there remains a need for an optical system for a laser scanning unit in which the optical components of one or more of the laser diode/pre-scan optical assemblies may be readily aligned. In particular, there is a need for methods and alignment structure for aligning the optical components of each laser diode/pre-scan optical assembly in which the location of the beam produced by the laser diode/pre-scan optical assembly may be accurately determined and in which the optical components comprising the laser diode/pre-scan optical assembly may be readily adjusted.